Hydraulic accu

A hydraulic accumulator. Picture: Les Chatfield.

During the second half of the nineteenth century, water motors were widely used in Europe and America. These small water turbines were connected to the tap and could power any machine that is now driven by electricity.

As we have seen in a previous article, operating motors with tap water was not very sustainable. Because of the low and irregular water pressure of the town mains, these motors used unacceptably high amounts of drinking water.

While the use of water motors in the US came to an end early in the twentieth century, the Europeans found a solution for the high water use of water motors and took hydraulic power transmission one step further.

They set up special "power water" networks, which distributed water under pressure for motive power purposes only, and switched to a much higher and more regular water pressure, made possible by the invention of the hydraulic accumulator.

Almost all these power water networks remained in service until the 1960s and 1970s. Hydraulic power transmission is very efficient compared to electricity when it is used to operate powerful but infrequently used machines, which can be distributed over a geographical area the size of a city.

The theoretical basis for hydraulic power transmission was laid in 1647 by French whizz-kid Blaise Pascal. By means of experiments, he discovered that water — unlike air — is virtually incompressible and transmits pressure equally in all directions.

The implications of the "hydrostatic paradox" were demonstrated in Pascal’s "machine for multiplying forces", illustrated below. It consists of two upright cylinders, connected together by a pipe. The whole system is filled with water and sealed water-tight. One cylinder contains a small diameter plunger, while the other cylinder contains a plunger that has a cross-sectional area 100 times larger.

Hydraulic jack blaise pascal

Machine for multiplying forces.

Pascal demonstrated that if a weight is placed on top of the small piston, it will be able to raise a weight placed on top of the larger piston that is 100 times heavier. Pascal’s machine thus allowed forces to be multiplied — in the example above, the  ratio of force output to force input is 100 to 1. In other words, you can produce an output force of 100 kg for an input force of only 1 kg.

A Machine for Multiplying Forces

Force multiplication was anything but new in the 1600s. More simple devices such as pulleys, gear trains, capstans, winches and treadwheels — all variations on the 7,000 year old lever — could also derive a high output force output from a small input force. For example, the Romans built cranes with a mechanical advantage of up to 70 to one, meaning that one man exerting a force of only 25 kg could raise a weight of 1.75 tonnes.

However, the hydraulic version of the lever has one outstanding advantage over earlier mechanisms: the friction loss is very small and independent of the mechanical advantage. Therefore, the possible multiplication ratio is almost infintely greater and both pistons may be a considerable distance apart — up to about 25 km, as we shall see.

Pascal's barrel experiment

Increasing force multiplication could be done by either extending the proportion between the diameter of both plungers, or by applying greater power to the smaller piston. In common with the earlier mechanisms, what is gained in mechanical advantage is lost in velocity ratio.

If a small hydraulic force is converted into a larger force, its speed of operation will be reduced in exactly the inverse proportion, because the distance traversed increases in the same proportion as the force. For example, a person pressing down the small piston 10 centimetres would move the other piston up only 1/100th of that distance.

Consequently, in a closed system, the heavier weight could be lifted only over a very limited distance, depending on the length of the plunger. However, this limit is removed when more water is added to the system and the smaller piston, instead of coming down just once, makes a number of strokes — in other words, when it functions as a pump. In this case, the larger piston will keep rising.

The Hydraulic Press

Pascal could only prove his point indirectly, as the available materials at the time were not strong enough to withstand the pressure. It would take another century and a half before hydraulic force multiplication was put into practice. Its first use was not a lifting device, but rather the opposite: the hydraulic press, which generates a compressive force.

The conventional screw press of the time, little developed since the Romans had used it for pressing olives and grapes, required a great effort to operate, had large frictional energy loss (+80%), and could not have exerted more than 25 tonnes load. (The screw, which converts rotational motion into linear motion, is basically an inclined plane wrapped around a cylinder).

Screw press versus hydraulic press

Left: The screw press. Picture credit: Bruce K. Satterfield. Right: The hydraulic press.

The hydraulic press was invented in 1796 by English locksmith and carpenter Joseph Bramah. It was entirely based on the theoretical work of Pascal. Bramah’s hydraulic press, which was driven by a hand-operated pump, brought a large increase in the load that could be exerted by a human.

With the available materials at the time, Bramah achieved an overall ratio of 1,000 to 1, which means that an effective load of 60 tonnes on the lifting piston could be balanced by a mere 60 kg on the pump handle. The efficiency of the hydraulic press was over 90%.

Harbours and Dockyards

In spite of its eminent suitability for crane operation, hydraulics made little progress in this field during the first half of the nineteenth century. This was largely due to the problem of reliably and efficiently translating the linear motion of a ram to rotary motion of the crane barrel or drum. During the first half of the nineteenth century, cargo handling in harbours, dockyards and railway yards was still done by means of human powered cranes, but the need for taller and stronger cranes was great.

Starting in the 1830s, iron began to be used as a material for ship building, with a parallel growth in the dimensions of ships. Conventional lifting systems were no longer adequate. In most countries, the solution was found in the steam powered crane, which appeared in the 1850s. However, in harbours and dockyards in Britain, a worthy alternative appeared: the water powered crane.

Hydraulic crane

British engineer William Armstrong started designing and operating powerful hydraulic cranes in the 1840s. Being fully aware that hydraulics was best adapted for giving a slow, steady motion, Armstrong deviced a method of lifting the load at one stroke of a ram or piston, multiplying the motion sufficiently by means of pulleys.

However, his efforts were complicated by the low and irregular pressure of the town mains, which was the power source for these machines. The maximum power output of a water powered machine is determined by water pressure and water flow. In the town mains, water pressure was (and often still is) supplied by a water tower. Because the practical height of a water tower is limited, so is the water pressure. A 50 m (165 ft.) tall water tower can produce a water pressure of 70 pound-per-square-inch (psi).

Consequently, the only way to further increase the power output of a crane running on water from the town mains is to increase the water flow. However, this raises potable water consumption and increases the size and costs of pipes, valves, cylinders, and other parts of the system. Moreover, if there is a higher than average demand for potable water from other users, the water level in a water tower will fall, and so will the water pressure and the power output of the machine.

The Hydraulic Accumulator

In 1851, Armstrong came up with an alternative solution that solved these issues: the hydraulic accumulator. Although much more compact than a water tower, it could produce a regular water pressure of 700 psi or higher — at least 10 times the water pressure in the town mains. This allowed to produce an order of magnitude more power without raising water consumption or increasing the size of system components.

Armstrong’s hydraulic accumulator was a contraption in which a ram or piston exerted pressure on the water in a vertical cylinder. The piston was loaded by dead weight ballast, which generally took on the form of a cylindrical ballast container surrounding the central cylinder (image below, on the left). The container was filled with crushed rock, scrap iron or other ballast material.

Hydraulic accumulators

Hydarulic Accumulator in Bristol Harbour. Wikipedia Commons.   Hydraulic Accumulator, Walsh Bay, Sydney. Source: NSW HSC Online.

For a water pressure of 700 psi the ballast was about 100 tonnes, acting on a ram of about 45 cm in diameter with a vertical stroke of 6 to 7 meters. Another type of accumulator utilised a rectangular platen to support a brickwork ballast (image above, on the right) or steel slabs. Hydraulic accumulators could be set up outdoors, or housed in a purpose designed building.

Fielding and platt hydraulic accumulator

The workings of the hydraulic accumulator are somewhat similar to those of a water tower. The central cylinder has a water inlet and outlet at the bottom. Water from the docks could be pumped in through the inlet by a steam powered pump, raising the piston, while it could be pushed out through the outlet into the mains for distribution, lowering the piston.

Energy was stored by upward movement of the ram and recovered upon its descent. The pumping rate of the steam engine was regulated in function of the water level in the accumulator, either automatically via mechanical linkages or via the aid of a human being.

Contrary to a water tower, however, the accumulator could maintain an even pressure all over the system regardless of the volume of water in the cylinder, because it’s the weight of the ballast and not the weight of the water that creates the pressure — in other words, the hydraulic accumulator gives pressure by load instead of by elevation.

With a charging/discharging efficiency above 98%, and no self-discharge, the hydraulic accumulator was an extremely energy efficient device.

Water Powered Factory Machinery

The introduction of the hydraulic accumulator had two important effects. First, it greatly expanded the range of hydraulically operated machines. The water motors connected to the town mains were household devices and workshop tools. But Armstrong and other engineers adapted high pressure water to a variety of industrial applications that required great power such as forging, punching, stamping, flanging, shearing and riveting (the predecessor of welding).

Hydraulic riveting press via old engineering

Hydraulically powered riveting machine.

In harbours, high pressure water not only operated cranes and hoisting machines handing cargo on docks and in warehouses, but also lock gates, swing bridges, boat lifts, and graving docks. At railway yards, hydraulic power transmission was used for freight handling and for moving railway cars (using hydraulic capstans), as well as for operating turntables, elevators and traversing mechanisms. All these applications of hydraulic power would have been impossible with the low and irregular pressure prevailing on the town mains.

To give an idea of the importance of hydraulic power, it suffices to look once more at the evolution of lifting devices. In 1586, a 344 ton obelisk was moved between squares in Rome. Domenic Fontana, master builder of the Vatican, raised the obelisk with the help of 40 capstans worked by 400 men and 75 horses. In 1878, John Dixon raised another obelisk — Cleopatra’s needle, weighing 209 tons — using four hydraulic lifting jacks, worked by four men.

Power Water Networks

Secondly, the hydraulic accumulator made it possible to transmit power efficiently over large distances. For a 30 cm diameter pipeline, the pressure drop in water distribution amounts to about 10 psi per mile, a figure that is independent of water pressure. Thus, if you transmit water with a pressure of 70 psi over a distance of 7 miles (12 km), all energy is lost. But if you transmit water over the same distance with a pressure of 700 psi, a water pressure of 630 psi remains, which comes down to a transmission efficiency of 90%.

The high transmission efficiency of high-pressure water led to the construction of at least a dozen public power water networks with accumulator storage, half of them in Britain, in which centrally located steam engines pumped water into hydraulic accumulators that distributed high pressure water over a large geographical area. One or more accumulators would be installed at each hydraulic power station and others could be sited at strategic points along the supply main as sub-stations.

From the 1870s to the 1890s, hydraulic power networks were established in the leading industrial cities of Britain: Kingston upon Hull, London, Liverpool, Birmingham, Grimsby, Manchester and Glasgow. Dock and railway companies pioneered the technology, and remained the most important users for decades.

Hydraulic accumulator hydraulic crane and hydraulic telescopic lift

Illustrations of a hydraulic accumulator, a hydraulic crane, and a hydraulic lift.

However, water power was also running manufacturing processes in factories, operating elevators in public, private and commercial buildings, and activating household devices and workshop tools. Anybody who was lucky enough to have a mains running through the street could connect to the public network. Power water consumption was metered, as happens today with potable water and electricity.

The idea of a truly hydraulic power network — analogous to the electric grid that came a bit later — was already outlined in a 1812 patent by Joseph Bramah, the inventor of the hydraulic press. But Bramah, who also conceived the hydraulic accumulator and the hydraulic crane, was ahead of his time. It took another sixty years before his ideas were brought into practice by Armstrong and his contemporaries.

London Hydraulic Power Company

The most extensive hydraulic power network was built in London, operated by the "London Hydraulic Company". At the company’s peak in 1917, five interconnected central power stations pumped high pressure water in about a dozen hydraulic accumulators and almost 300 km of supply mains, powering more than 8,000 machines and serving most of the city. In London theatres and other cultural buildings, power water was moving floors, organ consoles, fire curtains and stages. Water under pressure worked water pumps and lifted the bascules of the Tower Bridge.

Layout london hydraulic power company

Illustration: layout of London Hydraulic Power Co. mains and pumping stations, 1895.

Fire hydrants were also advantageously served by the high pressure system and several hundreds of them were connected to the London Hydraulic Power Company’s mains. These fire-fighting systems increased the pressure of the domestic water mains by injecting a small amount of high pressure water in them, using a jet pump. By itself, water at high pressure from the hydraulic power mains could not be supplied in adequate quantity to have an effect on a large fire, while the domestic supply mains had enough quantity but not enough pressure to reach the top floors of buildings.

Another remarkable application of high pressure water in London was the Silent Dustman, a water powered vacuum cleaning system that came on the market in 1910. Several large hotels were completely "wired" for this system: water from the town mains was used in a jet pump to produce a vacuum in a pipe to which the system was to be fitted. Along these pipes were a number of nozzles to which flexible hoses could be fixed. Thus the dirt from the sweepers was drawn into the hydraulic pipe and carried away into the drains. The system, which operated silently and efficiently, remained in operation until 1937.

The london hydraulic power company station wapping

One of the London power stations. Note the tower on the right, which houses the hydraulic accumulators.

In London, however, hydraulic power does not seem to have made a great impact on the domestic scene. In The Hydraulic Age (1980), B. Pugh notes that this was "possibly due to the fact that in its day domestic labour was cheap and in plentiful supply. Had present-day conditions operated then possibly the story would have been different since the potentialities of hydraulic power were not less than those of electricity today."

Most public power water networks supplied water under a pressure of 700 to 800 psi (48 to 55 bar), with the exception of Manchester and Glasgow, where water was pressurized to 1120 psi. In these cities, there was a heavy demand for power for hydraulic presses used for baling, an application that required a higher pressure.

Power Networks Outside Britain

The British power systems inspired similar networks elsewhere: Antwerp in Belgium, Buenos Aires in Argentina, and Melbourne and Sydney in Australia. While the Australian systems were reminiscent of those in Britain (with 80 km of mains, the one in Melbourne was the second largest ever built), the Argentinian system was used to pump sewage, and the network in Antwerp was aimed at the combined production of mechanical power and electricity. The latter was an attempt to overcome the very high transmission losses of electricity at the time.

Zuiderpershuis old picture

"Zuiderpershuis": a former hydraulic pumping plant in Antwerp. The towers housed the hydraulic accumulators.

In The Hydraulic Age, B. Pugh writes that:

"For power transmission, the early electric stations were faced with the same difficulties as the hydraulic power stations, their voltage being analogous to working pressure, and voltage drop due to mains resistance analogous to pressure drop due to pipe friction. The early electric public power stations were direct or continuous current stations, the voltage of generation essentially being only slightly higher (by the voltage drop in the cables) than at the consumer’s premises which for safety reasons had to be less than 250 volts. Due to voltage limitation, the area of supply as well as the amount of power that could be transmitted was limited."

Since 1865, Antwerp had been using a high pressure hydraulic network for powering cranes, bridges and sluices in the harbour. To this was added a second network in 1893, which distributed high pressure water to electric substations scattered across the city (twelve according to the plan, but only three were built). There, water turbines generated electricity which was distributed in a radius of 500 m via underground electric conduits — this was about the distance at which low voltage could be distributed efficiently.

Hydraulic cranes in antwerp

Hydraulic cranes in Antwerp harbour. Picture by Low-tech Magazine.

The Antwerp system, which was used for operating street lighting, thus did on a large scale what water motors connected to dynamos did on a small scale with water from the town mains (see the previous article). About 66% of the hydraulic energy was converted to electricity. At its peak, the network reached a length of 23 km with an output of 1200 hp. There were also a number of places in London where consumers ran small electric generators from the hydraulic supply.

Power Water Versus Electricity

The breakthrough in high voltage electric transmission at the turn of the century made systems like those in Antwerp immediately obsolete. The electricity generating part of the network disappeared in 1900. Producing water under pressure in order to produce electricity involves a fourfold energy conversion, which is needlessly wasteful if you can just produce electricity and transport it efficiently.

The expansion of efficient electrical transmission also stopped the construction of other large-scale power water networks before the century was over. "Had these systems been started some years earlier, they might have become vastly more popular", writes Ian McNeil in Hydraulic Power (1972). "A few years later, and they would probably never have been built at all."

However, almost all public power water systems that were built between the 1870s and 1890s remained in service until the 1960s and 1970s, eventually using electric motors instead of steam engines for pumping. The power water network operated by the London Hydraulic Company, the last to survive, worked until 1977. Most of the public power water networks kept growing during the first decades of the twentieth century, reaching their heydays at the end of the 1920s. The fatal decline came only when factories started leaving the cities in the 1960s and 1970s.

This raises two questions. First, why didn’t power water become the universal method of power distribution that Joseph Bramah and William Armstrong had envisioned? And second, if electricity is the most efficient and practical way of transmitting and distributing power, then why did almost all power water networks remain in service for almost a century?

Advantages of Electric Power

As a power transmission technology, power water has three important disadvantages in comparison to electricity. First of all, electricity can be transported efficiently over much longer distances. Hydraulic power transmission was (and still is) at least as efficient as electric power transmission up to distances of 15 to 25 km. Beyond those distances, however, electric transmission is a clear winner.

Hydraulic lock gate

Greenland dock hydraulic lock gates in London, built in the 1880s. Picture credit: Chris Allen.

A second shortcoming of hydraulic transmission is that a complex distribution network introduces additional energy loss. Every curve or bend in the mains increases friction losses. The more intricate the network, the less efficient it becomes. Electric transmission doesn’t have this problem, at least not in a significant way. The friction losses in the water mains limit the amount of machines that can be attached to a power water network, while electricity can be subdivided almost infinitely.

The third limitation of power water is the limited capacity of a hydraulic transmission line. Water under pressure can only be moved through thin pipes at walking speeds in order to avoid excessive friction losses. At higher speeds, the loss of friction increases as the square of the velocity and efficiency goes down fast, even over relatively short distances. This limits the flow rate and thus the power that could be delivered by a hydraulic transmission line.

Using a 10 to 12 cm diameter pipe — a common size in most high pressure system at the time — a hydraulic transmission line could produce a maximum continuous power of 115 to 205 horse power (85 to 150 kW). High voltage electric transmission lines of similar size can carry an amount of power that was orders of magnitude greater than that.

Advantages of Power Water

However, none of these disadvantages mattered for the power water networks that we have discussed. These were all decentralized systems, with machines no more than 15-25 km away from the power source.  Secondly, because the hydraulically operated machinery in harbours, railway yards, factories and buildings was characterized by slow motion and infrequent use, the slow transmission speed of power water presented no obstacle.

With the exception of the short-lived electricity generating system in Antwerp, none of the Armstrong-type power water networks supplied power to a large amount of continuously operating machines. (But note the medium pressure power water networks in Switzerland). Lastly, because a power water network operated relatively few (but very powerful) machines, friction loss through bends and curves in the network was limited.

Hydraulic accumulator

Hydraulic pump, accumulator and press. Source: Portefeuille économique des machines, de l’outillage et du matériel, December 1864, Bibliothèque nationale de France.

The limitations of hydraulic transmission were very well understood at the end of the nineteenth century. However, engineers also grasped the unique benefits of the technology, which still hold today. For example, Robert Zahner, an advocate of yet another alternative to electricity, compressed air, wrote in The Transmission of Power by Compressed Air (1890) that:

"The practical incompressibility of water renders the hydraulic method unfit for transmitting regularly a constant amount of power. It can be used to advantage only where motive power is to be accumulated and applied at intervals, such as raising weights, operating punches, compressive forging and other work of intermittent character, requiring a great force through a small distance."

Hydraulic transmission is "admirably adapted for use with heavy machinery and equipment in operations requiring marked concentration of power, reciprocating straight-line motion, and intermittent action", wrote Louis Hunter in The Transmission of Power (1991). The main excellence of the hydraulic accumulator is that it allows to operate machines that require much more power than the energy source can supply — Pascal’s "force multiplication".

Hydraulic lift

When high force or torque are needed, hydraulic power systems are a much more compact and energy efficient solution than mechanical or electric drives. Both electric motors and combustion engines often need mechanical power transmission (gears, chains, belts) to convert their high rotational speed to a slower speed with higher torque.

Likewise, hydraulic power systems easily produce linear motion using hydraulic cylinders, while electric power requires costly linear motors or mechanical power transmissions such as rack-and-pinion assemblies. Hydraulic and electric power are complementary in this sense: one of the limitations of power water transmission was the relative difficulty of converting linear motion to rotary motion.

Pelton wheels were the most obvious choice, but their high rotational speed involved the use of gearing for the operation of slow speed machinery. A number of hydraulic engines of the ram type was available to supply rotative power involving variable or slow speed operation, but these engines had few advantages compared to electric or mechanical drives.

A third important advantage of hydraulics is that the power is always readily available in the pipes and in the accumulator, but when there is no demand there is no waste. When none of the machines in a power water network was in operation, the hydraulic accumulators kept the lines pressurized without using any energy. This advantage is especially relevant when machines are used intermittently.

Hydraulics Today

Hydraulic power is still in use today, especially in heavy industrial equipment that requires a slow but powerful linear motion, and in mobile construction machinery such as excavators. However, the raised-weight hydraulic accumulator and the power water networks have disappeared.

The pressurized fluid is no longer water but oil, mixed with additives. (Vegetable oil had been used as a hydraulic medium in the 19th century). Unlike water, oil doesn’t freeze and is not corrosive. However, it makes hydraulic power more expensive and it obviously doesn’t permit the exhaust fluid to end up in the sewer network, the docks or the sea.

Partly as a consequence of the use of oil, there evolved the self-contained hydraulic power pack consisting of pump, hydraulic accumulator, and return flow systems, ready to be coupled to an electric motor or a diesel engine. The hydraulic accumulators in these systems are much smaller, they use a gas to compress the fluid, and they do not maintain a steady pressure.

Hydraulic accumulators today

Today’s hydraulic accumulators (usually compressed gas types) have little in common with the raised-weight accumulators in power water networks. Picture: HYD.

While the practical benefits of hydraulics remain — a large amount of power can be transferred and controlled precisely using very compact components — the modern approach erases an important efficiency advantage specific to the more centralized power water networks of the nineteenth and twentieth century. In a city-wide power water network, a comparably small central power source — a handful of hydraulic accumulators — could operate a large number of very powerful machines. The pumping engines didn’t have to be dimensioned for peak loads.

B. Pugh laments this evolution in The Hydraulic Age (1980):

"One century ago, only a few very large machines — swing bridges and an occasional hydraulic press — had their own individual pumping equipment. More recently, this trend spread throughout hydraulically operated machinery of all types and sizes, and is accepted practice today. With unit hydraulic power packs each piece of equipment will be driven by its own motor and will have its own instrumentation, filters, etcetera, which will call for periodic inspection and maintenance."

"The motor will run continuously while the unit is in use regardless of the load on the pump it drives. In the case of a number of such units not all will be working to capacity all the time. Appreciable economy could be effected by having a central pumping plant to supply a number of units and due to the diversification of the load the maximum load at any one time will be less than the sum of the individual maximum loads."

"An advantage of a large station over a number of smaller ones lies in the ability to meet diversity of demand. A number of small, independent power stations must each have sufficient capacity to meet the peak demand of its own area of supply and the peaks will not occur at the same time. A large station, embracing the total area of a number of small stations, will need only to meet the maximum simultaneous demand and this will normally be less than the sum total of the local peaks."

Alternatives to Electricity

Just like mechanical power transmission technologies — such as jerker line systems and endless rope drives — power water networks have disappeared largely because electric transmission has superior efficiency over long distances. However, in a more decentralized energy system based on renewable energy, all these forgotten alternatives for electricity deserve to be reconsidered for specific purposes. Raised-weight hydraulic accumulators could be solar, wind or even pedal powered.

Hydraulic power company valve cover

Picture: J.W. Gibson

Around 1900, the superiority of electricity for transmitting power over very long distances was not disputed. For moderate distances, however, quite a few authors doubted its usefulness. For example, R. Kennedy wrote in Modern Engines and Power Generators (1905):

"Electricity offers paramount advantages for power transmission to a distance in most cases. Electrical engineers, however, claim far too much for it. They are apt to forget other means for transmitting power, which means have paramount advantages over electricity in a good many cases."

W.C. Unwin, the author of the most complete nineteenth-century book on power transmission (On the Development and Transmission of Power from Central Stations), expressed a similar concern in 1894:

"Granting that electrical distribution will play an important part before long in the development of systems of power distribution, there is a popular tendency at the moment to regard too exclusively electrical methods, and to overlook other means of power distribution which have been usefully applied in the past, and will, in suitable conditions, be still employed in the future… For transmission to moderate distances there is a choice of several means of transmission, and electrical distribution has not in such cases and up to the present established any universal superiority."

In the next installment of our power transmission series, we will discuss compressed air, which is probably the most usable alternative for electricity.

Kris De Decker


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